C. Chromosome Numbering System and Linkage
Maps

Different systems for numbering chromosomes and linkage groups
of rice have been proposed by different workers as shown in Table 1. During
the International Rice Genetics Symposium held at IRRI in May 1985, attempts
were made to adopt a unified system of numbering the chromosomes and linkage
groups. For this purpose, an interim committee consisting of ten members and
chaired by Prof. C.M. Rick was appointed. The membership and report of this
interim committee follows this note. The consensus of the committee was that
chromosomes should be numbered in order according to their pachytene length.
Thus the numbering system of Shastry et al. (1960) which has been confirmed
by Kurata et al. (1981) was accepted. The linkage groups were associated with
respective chromosomes by Khush et al. (1984) and Iwata and Omura (1984) through
trisomic tests. However, some discrepancies between these two reports remain.
Dr. N. Kurata spent a couple of weeks at IRRI prior to International Rice Genetics
Symposium and examined the IRRI trisomic series. Her observations indicated
that there were differences in the identification of extra chromosomes of three
trisomics. However, there was insufficient time to resolve these differences
before the Symposium. It is hoped that Dr. N. Kurata will visit IRRI again in
near future to work with Dr. G.S. Khush in resolving these differences. Due
to the existing uncertainty in the association of linkage groups with respective
chromosomes, the current linkage maps are not presented in this volume but will
be reported in volume 3.

Table 1. Relationships among various systems of numbering linkage
groups, chromosomes and trisomics.

Report of Interim Committee on Chromosome Numbering
to the International Rice Genetics Symposium

May 29, 1985

In order to clarify understanding in rice genetics, a unified
system of numbering rice chromosomes needs to be adopted. The chromosomes shall
be assigned Arabic numbers in order according to their pachytene length (or
centromere position in case of ambiguity of length) based on studies by Dr.
S.V.S. Shastry et al. (1960) and confirmed by Dr. N. Kurata et al. (1981). The
somatic chromosomes shall be numbered accordingly. Complete primary trisomic
series have been established and the extra chromosomes identified in three laboratories
but differences exist in identification of extra chromosomes of a few trisomics.
These differences should be resolved as soon as possible. In this fashion all
linkage groups will be identified with their respective chromosomes.
Committee members:
C. M. Rick, Chairman, G. S. Khush, N. Kurata, H. I. Oka, R. Seetharaman, N.
Iwata, T. Kinoshita, R. N. Misra, R. Riley, H. K. Wu.